The said electromagnetic measuring probe allows the thickness of non-magnetic coatings or plating on magnetic substrate materials to be measured, for instance, a coat of paint on an iron or steel part.
All these measuring probes have an exciter winding which is connected to a constant alternating-current supply. Furthermore, there is an induction winding which generates a voltage U. The voltage in the induction winding is a measure of the thickness of the non-magnetic coating or plating.
This type of thickness measurement for thin coatings or platings should at least provide linearity of readout, i.e., a pointer should, for example, deflect twice as far for a coating or plating of double a certain thickness than it does for the said thickness alone.
It is highly desirable that thin coatings be measured without any influence from the substrate material on the characteristics curve. In this case, thin coatings are considered to be those which generate voltages lying within 0 to 1/3 of the normalized voltage, as defined later in the text.
It is, of course, a well known fact that the relationships between the generated voltage and the coating thickness are not linear for thin coatings or platings. It is, however, exactly in this range where linear measurements are most desirable. From the practical point of view, however, the range from 0 to roughly speaking, 50 micrometers is interesting as a lower range for coating and plating thicknesses.
Obviously, all these instruments must have facilities for switching ranges. If, for example, thicknesses up to 1000 micrometers are to be measured, and the said thickness is chosen as the upper end-scale value, then a thickness of 10 micrometers will be practically impossible to read.
Since it is impossible to avoid switching measuring ranges, then it is highly desirable that the said voltage is a linear function of coating or plating thickness not only at the lower end of the range, but also retains this same linearity to the highest possible thickness values, thus avoiding the necessity of using a different linear function in the upper ranges.
It must also be possible, of course, to make linear measurements in the lower range from 0 to 50 micrometers even when the probe is not handled with any particular care.
The conditions of measurement may change neither during a demonstration at a trade fair, nor over the long term in a harsh industrial environment. In the lower range in particular, the geometry of the crowned contact surface is an important factor. In accordance with Germany Utility Patent No. 73 36 864 an attempt was made to make the ball-like contact surface wear-resistant by depositing a titanium carbide coating onto it, i.e., to contrive that the geometry did not change. The coating, however, is very hard. It is deposited on a relatively soft material. If the thickness of the coating lies between 3 and 15 micrometers, it is possible that the titanium carbide coating will rupture when the probe is set down hard, because the substrate material underneath deforms.
It is also not very easy to deposit a uniformly thick coating on the substrate. If the coating is not uniformly thick, then different measured values are obtained depending on whether the probe is set down exactly at the centre of the ball-like contact surface or at another position. This realistic numerial example illustrates another problem associated with the measurement of thin coatings which, as yet, has no completely satisfactory solution.
A third problem must also be considered: The same probe must also be able to measure coatings on both flat surfaces and curved surfaces of small radius of curvature. An example of the latter is, e.g., a small nail, a small spring or similar.